The invention generally relates to electric motors and, more particularly, to electric motors for disk drives having fluid dynamic bearings.
Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft mounted on a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks. For example, to achieve increased storage densities, the read/write heads must be placed increasingly close to the surface of the storage disk. This proximity requires that the disk rotate substantially in a single plane. A slight wobble or run-out in disk rotation can cause the surface of the disk to contact the read/write heads. This is known as a xe2x80x9ccrashxe2x80x9d and can damage the read/write heads and surface of the storage disk, resulting is loss of data.
From the foregoing discussion, it can be seen that the bearing assembly that supports the storage disk is of critical importance. One bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil, or other fluids. The relatively rotating members may comprise bearing surfaces such as cones or spheres, or may alternately comprise fluid dynamic grooves formed on the members themselves. Fluid dynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This bearing surface distribution is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeatable run-out. Thus, fluid dynamic bearings are an advantageous bearing system.
However, current fluid dynamic bearing designs are susceptible to problems caused by tolerance-induced variations in bearing (and bearing groove) geometry. These problems include large variations in pressure produced in the journal bearing and the entrapment of air bubbles in the fluid itself. Both of the stated problems may be avoided by providing a fluid dynamic bearing with a fluid re-circulation path to an external environment. In addition to purging air from the bearing, re-circulation can also eliminate undesirable pressure variations caused by bearing geometry tolerances. Re-circulation to an external environment is difficult to achieve, however, in existing fluid dynamic bearing designs utilizing capillary seals because it would require the formation of multiple cross holes in the bearing sleeve. The ends of some of these holes would have to be plugged and sealed after formation. Capillary seals remain desirable, however, because they achieve sealing with minimal use of axial space and provide high shock resistance.
Therefore, a need exists for a fluid dynamic bearing design that utilizes capillary seals and can effectively and efficiently achieve fluid recirculation. Furthermore, a bearing design that can do so without foregoing some of the advantages of conventional bearings is also desirable.
The invention provides a fluid dynamic bearing motor featuring a stationary assembly supported from a base, a rotating assembly supported on the stationary assembly, a fluid dynamic bearing in a gap between the stationary and rotating assemblies, a radial capillary seal is defined proximate a first end of the gap, and at least one re-circulation hole extending from the radial capillary seal to a second end of the gap.